Visible laser projection systems are now widely used in industry to project a laser outline or “template” on a work or target surface for assembling large two or three-dimensional structures or assemblies, such as prefabricated roof trusses or aerospace composite components. By precisely characterizing the laser projector and establishing the exact relative position of the laser projector to the assembled structure or composite, the laser projection system is capable of accurately producing a laser image or template at known coordinates on a work or target surface which may be planar or curvilinear.
For example, U.S. Pat. No. 5,646,859 assigned in part to the assignee of this application, discloses a method and apparatus for defining a laser template for assembling a structure, such as a prefabricated roof truss. The method and apparatus disclosed in this patent includes a laser projector or a plurality of laser projectors mounted above the work surface, a plurality of laser sensors or laser targets fixed at predetermined locations on or adjacent the work surface, a computer and a sensor on the laser projector. The laser projector periodically or continuously scans the laser targets and the reflected light from the laser targets to the sensor of the laser projector determines the precise projection angle associated with the center of each target datum. Using a series of mathematical algorithms, the precise position and orientation of the laser projector relative to the work surface or part is then calculated by the computer. This spatial information, in conjunction with a known display list, allows the laser projector to generate accurate laser templates or laser outlines of the part on the target surface. The laser projector may be fixed relative to the part or work surface or for larger assemblies, a plurality of laser projectors may be used or the laser projectors may be moved relative to the work surface as disclosed in the above-referenced patent. The laser targets or position sensors may include a photo transistor, but in most applications, retroreflective laser targets are now used.
Alignment and calibration methods similar to the above provide the accuracy needed for a wide range of laser projection applications. A typical accuracy specification is ±0.015 inches at a 10 to 15 fool stand off distance when measured perpendicular to the laser beam. This approach allows good flexibility in positioning of the laser projectors because the mounting location can be arbitrarily selected so long as a sufficient number of known laser target locations are detectable within the field of view of the laser projector which, as set forth above, must be located at predetermined locations on or adjacent the target surface. In a typical application, a minimum of four laser targets must be located by the sensor system (laser target and sensor) to establish the position of the laser projector relative to the assembled structure or part and the work or target surface.
However, as set forth above, the requirement for laser targets at fixed locations on or adjacent the target surface has limited the applications for laser projection systems. For example, to guide the placement of carbon fiber composite materials for aerospace structures, it is generally necessary to locate the targets on the target surface which will eventually be covered by the composite material. Further, if the structure or target surface is very large, the required laser projection area may be too large to allow placement of laser or reference targets within the field of view of the laser projectors, making accurate projection difficult. Another problem addressed by the laser projection system of this invention is laser projection or image drift, which may result from variations in temperature and humidity. Further, the use of reference laser targets is cumbersome, initially requiring mounting of appropriate laser target locations on the target or tool together with accurate measurement of each target coordinate, typically using a theodelite laser tracker or other coordinate measuring machine (CMM). As used herein, CMM covers any coordinate measuring system. Further, when a laser projector is positioned or moved, the laser projector must be trained usually by manual operator guidance, to locale and identify the appropriate laser or reference targets and the placement of the laser projector must also be carefully controlled to ensure that the located laser targets provide an adequate position reference.
A disclosed embodiment of the laser projector system and method of this invention utilizes an indoor global positioning system (GPS), such as disclosed in U.S. Pat. Nos. 6,501,543 and 6,535,282 of Arc Second, Inc., the disclosures of which are incorporated herein by reference. Indoor GPS systems are also commercially available from Arc Second, Inc. However, the laser projection systems and methods of this invention are not limited to indoor GPS systems and other external metrology devices may be utilized, including laser theodelite transmitter tracking devices, optical photogrametry devices, camera base systems, infrared transmitter metrology devices and other metrology tracker projection devices. For example, Leica Geosystems and Northern Digital offer laser trackers systems for three-dimensional measurements, wherein the laser from the metrology transmitters is reflected by mirrors or a corner cube reflector (CCR) on a parallel path to a sensor on the metrology transmitters and the data from the receiver is transmitted to a computer to determine the location of the reflector. Such external metrology devices generally include a metrology transmitter, typically a light metrology transmitter, and a plurality of metrology receivers or reflectors which are fixed at predetermined locations.